"Preview" seems to have lost several files...

I often use the preview program to take notes for class, but when I went back to try to find these notes, the files were lost/inaccessible. If I right-click on the preview icon in the menu, I can still see the file listed, but it will not let me click on it or open it. I am also unable to search for these files in "finder". Is there any way to recover these files and get my notes back and is there a way to prevent Preview from doing this in the future?